1. Field of the Invention
The present invention relates to a semiconductor laser device including a multiple quantum well (MQW) active layer used in optical communication systems.
2. Description of the Related Art
Generally, a separate confinement heteroastructure (SCH) has been adopted, so as to confine electron-hole pairs and photons into well layers of an MQW active layer, thus improving the conversion efficiency. In addition, a graded index waveguide (GRIN)-SCH type III-IV group compound semiconductor laser device has been developed (see JP-A-61-258487, JP-A-6-45697 & JP-A-1-181492).
A prior art GRIN-SCH type semiconductor laser device includes an MQW active layer, and an n-type GRIN-SCH layer and a p-type GRIN-SCH layer sandwiching the MQW active layer. In this case, the energy gaps of the GRIN-SCH layers are larger than that of the barrier layers of the MQW active layer, which also causes linearly or curvilinearly graded infractive indexes in the GRIN-SCH layers. Thus, the linearly or curvilinearly energy gaps effectively confine electrons and holes in the MQW active layer, while the linearly or curvilinearly refractive indexes effectively confine photons in the MQW active layer. As a result, a conversion efficiency depending upon the interaction between electron-hole pairs and photons is enhanced, which improves the static characteristics of the semiconductor laser device. This will be explained later in detail.
The prior art semiconductor laser device, however, does not optimize the dynamic characteristics such as modulation operation characteristics, and as a result, it is impossible to obtain good modulation characteristics at high speed (high frequencies). In addition, since electrons and photons are confined in the narrow well layers of the MQW active layer, when the density of electrons therein is increased, free electrons are absorbed by the plasma effect of electrons, thus deteriorating the laser characteristics.